A composite video signal generally contains programming information and at least two other components: (1) luminance information (grayscale information generally sufficient to make a black and white image in substantially fall detail); and (2) chrominance information (coloration information). The luminance and chrominance information may partially share the same range of frequencies. Various electronic devices such as televisions and video cassette recorders generally contain a filter or other architecture for separating the luminance and chrominance information. In analog electronic devices, separation of luminance and chrominance information may be achieved using, in ascending order of typical desirability, a one-dimensional (“horizontal”) comb filter, a two-dimensional (“vertical”) comb filter, or a three-dimensional (“temporal”) comb filter.
Two-dimensional comb filters may enable substantial elimination of hanging dots by adaptively switching between horizontal and vertical comb filters based upon the content of a scene, for example. Three-dimensional comb filters may enable temporal comb filtering involving successive frames. Three-dimensional comb filters may, in some cases, achieve essentially perfect separation of luminance and chrominance information, eliminating substantially all dot crawl and rainbow swirls for stationary subject matter and may perform at least as well as two-dimensional comb filters for the rest of the image. Thus, it may be desirable to maximize use of a three-dimensional comb filter. In general, however, three-dimensional comb filters may achieve the best results when used for still frames or for those portions of consecutive frames not affected by motion or change of scene because there may be some loss of quality, such as dot crawl or hanging dots, as the subject moves from one frame to the next. If there is subject motion, comb filter logic, such as a motion detector, may sense the existence of motion and switch from use of a three-dimensional comb filter to use of the less-desirable two-dimensional comb filter.